Allotropy is the existence of one element in more than one form. These have the same chemical properties but the different physical properties. The various forms of elements are called allotropes. As we know that carbon has three allotropes are named as:- Diamond, graphite and fullerene
Inert pair effect-
On moving down the group in the periodic table, the tendency of -orbital electron to participate in bonding is decreased. This effect is known as the inert pair effect. For example, in group 13 element ()the stability of +1 oxidation is more than the +2 oxidation state due to the poor shielding of the electrons by the and electron, as a result, electrons are strongly held by the nucleus.